5

ATA5723C/ATA5724C/ATA5728C [DATASHEET]

9248D–RKE–10/14

3.

RF Front-end

The RF front-end of the receiver is a low-IF heterodyne configuration that converts the input signal into about 1MHz IF signal

with a typical image rejection of 30dB. According to Figure Figure 1-2 on page 3 the front-end consists of an LNA (low noise

amplifier), LO (local oscillator), I/Q mixer, polyphase low-pass filter and an IF amplifier.

LC-VCO (voltage controlled oscillator) and PLL-loop filter. The XTO (crystal oscillator) generates the reference frequency

the following formula:

The XTO is a two-pin oscillator that operates at the series resonance of the quartz crystal with high current but low voltage

signal, so that there is only a small voltage at the crystal oscillator frequency at pins XTAL1 and XTAL2. According to

The value of these capacitors are recommended by the crystal supplier. Due to an inductive impedance at steady state

best way is to use a crystal with known load resonance frequency to find the right value for this capacitor.) When designing

the system in terms of receiving bandwidth and local oscillator accuracy, the accuracy of the crystal and the XTO must be

considered.

Figure 3-1. XTO Peripherals

(low-side injection):

The relationship is designed to achieve the nominal IF frequency of:

The RF input either from an antenna or from an RF generator must be transformed to the RF input pin LNA_IN. The input

impedance of this pin is provided in the electrical parameters. The parasitic board inductances and capacitances influence

the input matching. The RF receiver Atmel ATA5723C/ATA5724C/ATA5728C exhibits its highest sensitivity if the LNA is

power matched. Because of this, matching to a SAW filter, a 50

Ω or an antenna is easier. Figure 14-1 on page 29 shows a

shown in Table 14-2 on page 29 is the reference network for the parameters given in the electrical characteristics.

XTAL2

TEST2

TEST3

XTAL1

DVCC